fix(auth): address PR #758 round-4 review

Seven findings from the latest review on #758 (3 medium, 4 low/nit):

Medium:
- storage.py: replace 5 ``assert self.cipher is not None`` sites with
  explicit ``RuntimeError`` so missing TOKEN_ENCRYPTION_KEY can't silently
  become an AttributeError under ``python -O``
- session_backend.py: document the silent-invalidation invariant —
  refresh-token TTL expiry without explicit logout deliberately makes
  the browser session unusable; future readers must not relax it
- server/oauth_tools.py: drop user_id from the Flow 2 session_id
  identifier — use ``flow2_{secrets.token_hex(16)}`` so audit logs and
  DB rows don't carry user_id in the session_id field

Low / nit:
- token_utils.py: drop _fetch_locks dict entry in finally so a probed
  deployment can't grow the lock dict without bound; coalescing test
  now pins the invariant with len(_fetch_locks) == 0
- browser_oauth_routes.py: strip trailing slash from settings.nextcloud_host
  before constructing the well-known URL so a host configured as
  ``https://cloud.example.com/`` doesn't produce a double-slash
- browser_oauth_routes.py: add comment explaining the three-layer CSRF
  policy on the mcp_session cookie set (SameSite=Lax + POST-only logout
  + Origin/Referer check)
- oauth_routes.py: convert all 23 f-string log calls to lazy %-style
  per the CLAUDE.md / memory feedback_lazy_logging convention

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/test_id_token_verification.py

@@ -528,3 +528,10 @@ async def test_get_cached_coalesces_concurrent_misses():
     assert all(r == results[0] for r in results), (
         "concurrent callers received divergent cached data"
     )
+    # Pin the round-4 cleanup invariant: _fetch_locks must drain after the
+    # fetch completes so a probed deployment can't accumulate locks for
+    # arbitrary URLs.
+    assert len(token_utils._fetch_locks) == 0, (
+        "expected _fetch_locks to be empty after fetch, "
+        f"found {list(token_utils._fetch_locks)}"
+    )